1. Field of the Invention
The present invention relates to a roll printing device for coating an alignment layer of a liquid crystal display (LCD) device, and more particularly, to a roll printing device that can uniformly coat an alignment layer.
2. Discussion of the Related Art
Among ultra thin flat panel display devices having a display screen of a thickness of several centimeters, LCD devices have been widely used for notebook computers, monitors, spaceships, aircrafts, etc. due to their advantageous features of low driving voltage, low power consumption, and portability.
The LCD device includes a lower substrate, an upper substrate, and a liquid crystal layer formed between the substrates, wherein the lower substrate is formed opposite the upper substrate at a predetermined interval. Alignment of the liquid crystal layer depends on whether a voltage is applied between the two substrates, and transmittance of light depends on the alignment of the liquid crystal layer, whereby an image is displayed. At this time, if the alignment of the liquid crystal layer is disordered, it is difficult to obtain a desired image. Accordingly, an alignment layer is formed to uniformly maintain an initial alignment of the liquid crystal layer.
Examples of a method for forming such an alignment layer include a rubbing alignment method and a photo-alignment method.
In the rubbing alignment method, after an alignment layer is thinly coated on a substrate, a rubbing roll wound with a rubbing cloth is rotated to arrange the alignment layer in a constant direction.
In the photo-alignment method, after an alignment layer is thinly coated on a substrate, ultraviolet (UV) rays such as polarized light or unpolarized light are irradiated toward the alignment layer to allow the alignment layer to react with the UV rays, thereby arranging the alignment layer in a constant direction.
In order to obtain an alignment layer aligned in a constant direction using the rubbing alignment method or the photo-alignment method, it is necessary to thinly coat the alignment layer on the substrate at a uniform thickness. To this end, a roll printing method has been conventionally used.
Hereinafter, a related art roll printing method will be described with reference to the accompanying drawings.
FIG. 1A is a sectional view illustrating a method of coating an alignment layer using a related art roll printing device.
First, after a structure of a related art roll printing device is described, process steps of coating an alignment layer using the roll printing device will be described.
As shown in FIG. 1A, the related art roll printing device includes a dispenser 14, a doctor roll 16, an anilox roll 18, and a printing roll 20.
The doctor roll 16 is rotated in engagement with the anilox roll 18, and the anilox roll 18 is rotated in engagement with the printing roll 20.
The printing roll 20 is attached with a printing mask 22 suitable for a coating pattern of a desired alignment material.
A method of coating an alignment layer using the aforementioned roll printing device will now be described.
First, the dispenser 14 supplies an alignment material 15 to the anilox roll 18. The alignment material supplied to the anilox roll 18 is coated on the printing mask 22 of the printing roll 20 rotating in engagement with the anilox roll 18.
A substrate 10 is mounted on a substrate stage 12. The substrate stage 12 moves in a constant direction below the printing roll 20. While the substrate stage 12 is moving, the substrate 10 mounted on the substrate stage 12 is in contact with the printing mask 22 attached to the printing roll 20 and the alignment material coated on the printing mask 22 is transferred onto the substrate 10. Thus, the alignment material is coated on the substrate 10.
The doctor roll 16 serves to uniformly coat the alignment material 15 supplied from the dispenser 14 on the anilox roll 18 while rotating in engagement with the anilox roll 18.
However, the related art roll printing method for realizing uniform coating of the alignment material using the doctor roll 16 has several problems.
First, as the size of the substrate increases, the size of the anilox roll 18 increases along with the size of the doctor roll 16 rotating in engagement with the anilox roll 18. In this case, a problem occurs in that the alignment material is not uniformly coated on a center portion of the anilox roll 18.
Referring to FIG. 1B, if the sizes of the doctor roll 16 and the anilox roll 18 increase, pressure between the doctor roll 16 and the anilox roll 18 decreases in a center portion (portion B) of the rolls 16 and 18 more seriously than the pressure between the end portions (portions A) of the rolls 16 and 18. Accordingly, the alignment material is not uniformly coated on the center portion (portion B) of the anilox roll 18.
Second, if coating is repeated, a predetermined portion of the doctor roll 16 is abraded. In this case, the alignment material is not uniformly coated on the abraded portion. For this reason, a problem occurs in that a spot is generated on a predetermined portion of the substrate.
In other words, since the doctor roll 16 is made of a rubber based material, it is abraded by the alignment material if coating is repeated, whereby the alignment material is not uniformly coated on the doctor roll 16.